1. Field of the Invention
The present invention relates to fuel cells, and in particular intermediate-temperature solid oxide fuel cells (IT-SOFCs) which are typically used in stacks to generate a power output of from 1 to 100 kW and find application as local power generators, for example, in remote locations, such as for residential combined heat and power (CHP) generation, and in vehicles, either as a primary power unit (PPU), an auxiliary power unit (APU) or to drive other equipment, such as air-conditioning equipment.
For solid oxide fuel cells other than those integrated with a gas turbine, the fuel cells should be operated at the lowest temperature possible without compromising the electrode kinetics and electrolyte resistance.
2. Description of the Prior Art
Using known fabrication routes in the fabrication of ceramic electrolytes, it is generally accepted that the minimum film thickness that can be reliably fabricated is about 10 xcexcm. This minimum electrolyte thickness establishes a minimum operating temperature, typically about 650xc2x0 C. for scandia-stabilised zirconia (SSZ) electrolytes, about 700xc2x0 C. for yttria-stabilised zirconia (YSZ) electrolytes, and about 500xc2x0 C. for doped ceria electrolytes, such as gadolinia-doped ceria (CGO) electrolytes. Further, the use of such a thin electrolyte film requires a substrate in order to provide a fuel cell having the necessary robustness.
For zirconia-based electrolytes, for example YSZ, a porous NiOxe2x80x94YSZ anode substrate typically having a thickness in the range of from 250 to 500 xcexcm is commonly used. Numerous techniques have been used to deposit electrolyte films on substrates. These techniques include screen printing, tape casting, vacuum slip casting, electrophoretic deposition, calendering, spray pyrolysis, sputtering and plasma spraying. In such fuel cells, the electrolyte film and the substrate are usually co-fired at high temperature, typically about 1400xc2x0 C., to ensure that the electrolyte film is dense and impermeable to gaseous molecules.
Whilst NiOxe2x80x94YSZ/YSZ structures have been successfully fabricated, the use of an NiOxe2x80x94YSZ substrate does give rise to a number of problems. These problems include poor thermal expansion compatibility, NiOxe2x80x94YSZ having a coefficient of thermal expansion in the range of 12 to 13xc3x9710xe2x88x926 Kxe2x88x921 as compared to 10.6xc3x9710xe2x88x926 Kxe2x88x921 for YSZ. NiOxe2x80x94Al2O3 and NiOxe2x80x94TiO2 substrates, which do have an improved thermal expansion match, are being developed, but these substrates still require a thin active interfacial layer of NiOxe2x80x94YSZ between the substrate and the electrolyte film to promote the electrochemical oxidation of the fuel. Another problem associated with the use of an NiOxe2x80x94YSZ substrate is the volume change associated with the reduction of the NiO component to Ni when in contact with the gaseous fuel. This volume change weakens the substrate and requires the fuel to be initially introduced very slowly into the stack to accommodate the volume change. Furthermore, with the use of an NiOxe2x80x94YSZ substrate, it is essential to ensure that the anode compartment remains sufficiently reducing so as to ensure that the Ni is not oxidised back to NiO, particularly during any cooling cycles.
Owing in part to the above-mentioned disadvantages of the ceramic NiOxe2x80x94YSZ substrate, the use of porous metallic substrates has been proposed, as disclosed, for example, in GB-A-1049428. The principal advantages of metallic substrates are recognised as the excellent mechanical behaviour and the improved electrical and thermal conductivity. However, the use of metallic substrates constrains the maximum fabricating temperature to about 1000xc2x0 C., which temperature is below that required to sinter supported zirconia-based electrolytes into a dense impermeable film. Also, it is necessary to seal around the periphery of the porous substrate to prevent mixing of the gaseous oxidant and fuel. Currently, brittle glass, glass-ceramic or composite metal/ceramic seals are used for this purpose, which seals often crack during the thermal cycling experienced during operation.
As a consequence of the limitation to the fabrication temperature introduced by using metallic substrates, GB-A-1049428 discloses the use of plasma spraying to prepare dense films of zirconia-based electrolytes. Whilst plasma spraying can be used to deposit electrolyte films, that deposition technique is relatively expensive, in particular being wasteful of the expensive ceramic powder. Other physical vapour deposition (PVD) techniques have also been used to deposit thin electrolyte films, but these techniques are also relatively expensive and not as convenient as the conventional ceramic processing routes. Chemical vapour deposition (CVD) techniques have also been used to deposit thin electrolyte films, but these techniques are still more expensive and likewise not as convenient as the conventional ceramic processing routes.
Alternative fuel cell designs have also been proposed, such as the circular fuel cell design as disclosed, for example, in U.S. Pat. Nos. 5,368,667, 5,549,983 and 5,589,017. In this circular design, the gaseous oxidant and fuel are introduced via a manifold at the centre of the fuel cell stack, and the distribution and flow rate of the gaseous oxidant and fuel are arranged such as to ensure almost complete conversion of the fuel prior to reaching the periphery of the stack. With this design, only one brittle glass or glass-ceramic seal is required at the central manifold as the excess oxidant and fuel are combusted at the periphery of the stack. Although this fuel cell design represents an improvement, the brittle glass, glass-ceramic or composite metal/ceramic seal required at the central manifold is still liable to crack during the rapid thermal cycling experienced during operation. Moreover the maximum diameter of this circular design SOFC is typically limited to about 15 cms due to fabrication constraints. Accordingly the electrical power than can be generated within a single stack is limited.
It is thus an aim of the present invention to provide a solid oxide fuel cell and a method of fabricating the same which utilises a metallic substrate, enables the fabrication of a ceramic electrolyte film by sintering, and avoids the need to use brittle seals.
Accordingly, the present invention provides a solid oxide fuel cell, comprising: a ferritic stainless steel substrate including a porous region and a non-porous region bounding the porous region; a ferritic stainless steel bi-polar plate located under one surface of the porous region of the substrate and being sealingly attached to the non-porous region of the substrate about the porous region thereof; a first electrode layer located over the other surface of the porous region of the substrate; an electrolyte layer located over the first electrode layer; and a second electrode layer located over the electrolyte layer.
Preferably, the ferritic stainless steel is a ferritic stainless steel containing no aluminium.
Preferably, the ferritic stainless steel is a titanium/niobium stabilised ferritic stainless steel.
More preferably, the ferritic stainless steel contains from about 17.5 to 18.5 wt % Cr (European designation 1.4509).
Preferably, the substrate has a thickness of from about 50 to 250 xcexcm.
More preferably, the substrate has a thickness of from about 50 to 150 xcexcm.
Yet more preferably, the substrate has a thickness of about 100 xcexcm.
Preferably, the porous region of the substrate includes a plurality of through apertures fluidly interconnecting the one and other surface of the substrate.
More preferably, the apertures are uniformly spaced.
Preferably, the apertures have a lateral dimension of from about 5 to 250 xcexcm.
More preferably, the apertures have a lateral dimension of from about 20 to 50 xcexcm.
Yet more preferably, the apertures have a lateral dimension of about 30 xcexcm.
Preferably, the apertures comprise from about 30 to 65 area % of the porous region of the substrate.
More preferably, the apertures comprise from about 50 to 55 area % of the porous region of the substrate.
Preferably, the substrate includes an active coating of an electronically-conductive oxide.
In one embodiment the active coating is a perovskite oxide mixed conductor.
Preferably, the perovskite oxide mixed conductor comprises La1-xSrxCOyFe1-yO3-xcex4, where 0.5xe2x89xa7xxe2x89xa70.2 and 0.3xe2x89xa7yxe2x89xa70.
More preferably, the perovskite oxide mixed conductor comprises one of La0.6Sr0.4Co0.2Fe0.8O3-xcex4, La0.5Sr0.5CoO3-xcex4, Gd0.5CoO3-xcex4. and Sm0.5Sr0.5CoO3-xcex4.
In another embodiment the active coating comprises doped LaMnO3.
In one embodiment the substrate includes a recess in which the first electrode layer is at least partially located.
Preferably, the substrate comprises a foil.
Preferably, the substrate is a photo-chemically machined and/or laser machined substrate.
In other embodiments the substrate could be composed of a porous sintered metal powder region joined to a non-porous region. The thickness of such a sintered metal powder substrate would typically be in the region of 250 to 1000 xcexcm.
Preferably, one or both of the first and second electrode layers has a thickness of from about 10 to 25 xcexcm.
More preferably, one or both of the first and second electrode layers has a thickness of from about 10 to 15 xcexcm.
Preferably, one or both of the first and second electrode layers is a sintered material.
In a preferred embodiment one of the first and second electrode layers comprises a sintered powdered mixture of perovskite oxide mixed conductor and rare earth-doped ceria.
Preferably, the powdered mixture comprises about 60 vol % of perovskite oxide mixed conductor and about 40 vol % of rare earth-doped ceria.
Preferably, the perovskite oxide mixed conductor comprises La1-xSrxCoyFe1-yO3-xcex4, where 0.5xe2x89xa7xxe2x89xa70.2 and 1xe2x89xa7yxe2x89xa70.2.
More preferably, the perovskite oxide mixed conductor comprises one of La0.6Sr0.4Co0.2Fe0.8O3-xcex4, La0.5Sr0.5CoO3-xcex4, Gd0.5CoO3-xcex4. and Sm0.5Sr0.5CoO3-xcex4.
Preferably, the rare earth-doped ceria comprises Ce1-xRExO2-x/2, where RE is a rare earth and 0.3xe2x89xa7xxe2x89xa70.05.
More preferably, the rare earth-doped ceria comprises Ce0.9Gd0.1O1.95.
In one embodiment the one of the first and second electrode layers is the first electrode layer provided as a cathode layer.
In a preferred embodiment the other of the first and second electrode layers comprises a sintered powdered mixture of NiO and rare earth-doped ceria.
Preferably, the powdered mixture comprises about 50 vol % of NiO and about 50 vol % of rare earth-doped ceria or un-doped ceria.
Preferably, the rare earth-doped ceria comprises Ce1-xRExO2-x/2, where RE is a rare earth and 0.3xe2x89xa7xxe2x89xa70.05.
More preferably, the rare earth-doped ceria comprises Ce0.9Gd0.1O1.95.
In a particularly preferred embodiment the other of the first and second electrode layers is the second electrode layer provided as an anode layer.
Preferably, the electrolyte layer has a thickness of from about 5 to 30 xcexcm.
In one embodiment the electrolyte layer comprises a sintered powdered mixture of rare earth-doped ceria and cobalt oxide.
Preferably, the sintered powdered mixture comprises about 98 mole % rare earth-doped ceria and about 2 mole % cobalt oxide.
Preferably, the rare earth-doped ceria comprises Ce1-xRExO2-x/2, where RE is a rare earth and 0.3xe2x89xa7xxe2x89xa70.05.
More preferably, the rare earth-doped ceria comprises Ce0.9Gd0.1O1.95.
In another embodiment the electrolyte layer comprises a sintered layer of doped ceria.
The present invention further provides a fuel cell stack comprising a plurality of the above-described fuel cells.
The power output and scalability of the fuel cell are improved in preferred embodiments in which an array of elements each comprising a first electrode layer, an electrolyte layer and a second electrode layer are provided upon said substrate.
In a preferred embodiment the present invention avoids the need to use brittle seals by using a metal foil substrate including a porous region fabricated by photo-chemical machining and cell compositions that allow operation at 500xc2x0 C. or below. This relatively low operating temperature allows the use of commercially available compliant gaskets to seal the internal manifold configuration incorporated in the bi-polar plates.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.